MV design guide - Schneider Electric

Design rules
Dielectric withstand
The shape of parts
This plays a key role in switchgear dielectric withstand.
It is essential to eliminate any "peak" effect which would have a disastrous
effect on the impulse wave withstand in particular and on the surface
ageing of insulators:
Air ionization Ozone production Breakdown of moulded insulator surface skin
Distance between parts
Ambient air between live parts
c For installations in which, for various reasons, we cannot test under
impulse conditions, the table in publication IEC 71-2 gives, according to
the rated lightning impulse withstand voltage, the minimum distances to
comply with in air either phase to earth or phase to phase.
V 0
d
c These distances guarantee correct withstand for unfavourable
configurations: altitude < 1 000 m.
c Distances in air* between conductive parts that are live and structures
which are earthed giving a specified impulse withstand voltage under dry
conditions:
U
Rated lightning
impulse withstand
voltage
Up (kV)
40 60
60 90
75 120
95 160
125 220
Minimum distance
in air phase
to earth and phase
to phase
d (mm)
The values for distances in air given in the table above are minimum
values determined by considering dielectric properties, they do not
include any increase which could be required to take account of design
tolerances, short circuit effects, wind effects, operator safety, etc.
*These indications are relative to a distance through a single air gap, without taking account of
the breakdown voltage by tracking across the surfaces, related to pollution problems.
Schneider Electric
Merlin Gerin MV design guide
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